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  JUC-17-ConcurrentHashMap
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      <h1 id="JUC-17-ConcurrentHashMap"><a href="#JUC-17-ConcurrentHashMap" class="headerlink" title="JUC-17-ConcurrentHashMap"></a>JUC-17-ConcurrentHashMap</h1><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><ul>
<li><code>ConcurrentHashMap</code> 是一个并发散列映射表，它允许完全并发的读取，并且支持给定数量的并发更新。</li>
<li>而<code>HashTable</code>和同步包装器包装的 <code>HashMap</code>，使用一个全局的锁来同步不同线程间的并发访问，同一时间点，只能有一个线程持有锁，也就是说在同一时间点，只能有一个线程能访问容器，这虽然保证多线程间的安全并发访问，但同时也导致对容器的访问变成串行化的了。</li>
</ul>
<a id="more"></a>

<p><a href="https://blog.csdn.net/weixin_44460333/article/details/86770169" target="_blank" rel="noopener">https://blog.csdn.net/weixin_44460333/article/details/86770169</a></p>
<p><a href="https://www.jianshu.com/p/d0b37b927c48" target="_blank" rel="noopener">https://www.jianshu.com/p/d0b37b927c48</a></p>
<p><a href="https://zhuanlan.zhihu.com/p/21673805" target="_blank" rel="noopener">https://zhuanlan.zhihu.com/p/21673805</a></p>
<p><a href="https://www.jianshu.com/p/865c813f2726" target="_blank" rel="noopener">https://www.jianshu.com/p/865c813f2726</a></p>
<h2 id="1-JDK-1-7-概述"><a href="#1-JDK-1-7-概述" class="headerlink" title="1. JDK 1.7 概述"></a>1. JDK 1.7 概述</h2><ul>
<li>在JDK1.7版本中，ConcurrentHashMap的数据结构是由一个Segment数组和多个HashEntry组成，如下图所示：</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200921/203834721.png" alt="mark"></p>
<ul>
<li>Segment数组的意义就是将一个大的table分割成多个小的table来进行加锁，也就是上面的提到的锁分离技术，而每一个Segment元素存储的是<code>HashEntry</code>数组+链表，这个和HashMap的数据存储结构一样</li>
</ul>
<h2 id="2-JDK-1-8-实现"><a href="#2-JDK-1-8-实现" class="headerlink" title="2. JDK 1.8 实现"></a>2. JDK 1.8 实现</h2><ul>
<li>JDK1.8的实现已经摒弃了Segment的概念，而是直接用Node数组+链表+红黑树的数据结构来实现</li>
<li><strong>并发控制使用Synchronized和CAS来操作</strong>， 整个看起来就像是优化过且线程安全的HashMap</li>
<li>虽然在JDK1.8中还能看到Segment的数据结构，但是已经简化了属性，只是为了兼容旧版本</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200921/211002824.png" alt="mark"></p>
<p>说明：ConcurrentHashMap的数据结构（数组+链表+红黑树），桶中的结构可能是链表，也可能是红黑树，红黑树是为了提高查找效率。</p>
<ul>
<li>在深入JDK1.8的put和get实现之前要知道一些常量设计和数据结构，这些是构成ConcurrentHashMap实现结构的基础，<strong>下面看一下基本属性：</strong></li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br></pre></td><td class="code"><pre><span class="line">  &#x2F;&#x2F; node数组最大容量：2^30&#x3D;1073741824  </span><br><span class="line"></span><br><span class="line">  private  static  final  int  MAXIMUM_CAPACITY &#x3D;  1  &lt;&lt;  30    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 默认初始值，必须是2的幂数  </span><br><span class="line"></span><br><span class="line">  private  static  final  int  DEFAULT_CAPACITY &#x3D;  16    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F;数组可能最大值，需要与toArray（）相关方法关联  </span><br><span class="line"></span><br><span class="line">  static  final  int  MAX_ARRAY_SIZE &#x3D; Integer.MAX_VALUE -  8    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F;并发级别，遗留下来的，为兼容以前的版本  </span><br><span class="line"></span><br><span class="line">  private  static  final  int  DEFAULT_CONCURRENCY_LEVEL &#x3D;  16    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 负载因子  </span><br><span class="line"></span><br><span class="line">  private  static  final  float  LOAD_FACTOR &#x3D;  0    .75f;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 链表转红黑树阀值,&gt; 8 链表转换为红黑树  </span><br><span class="line"></span><br><span class="line">  static  final  int  TREEIFY_THRESHOLD &#x3D;  8    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F;树转链表阀值，小于等于6（tranfer时，lc、hc&#x3D;0两个计数器分别++记录原bin、新binTreeNode数量，&lt;&#x3D;UNTREEIFY_THRESHOLD 则untreeify(lo)）  </span><br><span class="line"></span><br><span class="line">  static  final  int  UNTREEIFY_THRESHOLD &#x3D;  6    ;  </span><br><span class="line"></span><br><span class="line">  static  final  int  MIN_TREEIFY_CAPACITY &#x3D;  64    ;  </span><br><span class="line"></span><br><span class="line">  private  static  final  int  MIN_TRANSFER_STRIDE &#x3D;  16    ;  </span><br><span class="line"></span><br><span class="line">  private  static  int  RESIZE_STAMP_BITS &#x3D;  16    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 2^15-1，help resize的最大线程数  </span><br><span class="line"></span><br><span class="line">  private  static  final  int  MAX_RESIZERS &#x3D; (    1  &lt;&lt; (    32  - RESIZE_STAMP_BITS)) -  1    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 32-16&#x3D;16，sizeCtl中记录size大小的偏移量  </span><br><span class="line"></span><br><span class="line">  private  static  final  int  RESIZE_STAMP_SHIFT &#x3D;  32  - RESIZE_STAMP_BITS;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; forwarding nodes的hash值  </span><br><span class="line"></span><br><span class="line">  static  final  int  MOVED     &#x3D; -    1    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 树根节点的hash值  </span><br><span class="line"></span><br><span class="line">  static  final  int  TREEBIN   &#x3D; -    2    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; ReservationNode的hash值  </span><br><span class="line"></span><br><span class="line">  static  final  int  RESERVED  &#x3D; -    3    ;  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F; 可用处理器数量  </span><br><span class="line"></span><br><span class="line">  static  final  int  NCPU &#x3D; Runtime.getRuntime().availableProcessors();  </span><br><span class="line"></span><br><span class="line">  &#x2F;&#x2F;存放node的数组  </span><br><span class="line"></span><br><span class="line">  transient  volatile  Node&lt;K,V&gt;[] table;  </span><br><span class="line"></span><br><span class="line">  &#x2F;*控制标识符，用来控制table的初始化和扩容的操作，不同的值有不同的含义  </span><br><span class="line"></span><br><span class="line">  *当为负数时：-    1    代表正在初始化，-N代表有N-    1    个线程正在 进行扩容  </span><br><span class="line"></span><br><span class="line">  *当为    0    时：代表当时的table还没有被初始化  </span><br><span class="line"></span><br><span class="line">  *当为正数时：表示初始化或者下一次进行扩容的大小  </span><br><span class="line">*&#x2F;</span><br><span class="line"></span><br><span class="line">  private  transient  volatile  int  sizeCtl;</span><br></pre></td></tr></table></figure>

<p>基本属性定义了ConcurrentHashMap的一些边界以及操作时的一些控制，下面看一些内部的一些结构组成，这些是整个ConcurrentHashMap整个数据结构的核心</p>
<h3 id="2-1-Node"><a href="#2-1-Node" class="headerlink" title="2.1 Node"></a>2.1 Node</h3><ul>
<li>Node是ConcurrentHashMap存储结构的基本单元，继承于HashMap中的Entry，用于存储数据，源代码如下</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">Node</span>&lt;<span class="title">K</span>,<span class="title">V</span>&gt; <span class="keyword">implements</span> <span class="title">Map</span>.<span class="title">Entry</span>&lt;<span class="title">K</span>,<span class="title">V</span>&gt; </span>&#123;</span><br><span class="line">    <span class="keyword">final</span> <span class="keyword">int</span> hash;</span><br><span class="line">    <span class="keyword">final</span> K key;</span><br><span class="line">    <span class="keyword">volatile</span> V val;</span><br><span class="line">    <span class="keyword">volatile</span> Node&lt;K,V&gt; next;</span><br><span class="line"></span><br><span class="line">    Node(<span class="keyword">int</span> hash, K key, V val, Node&lt;K,V&gt; next) &#123;</span><br><span class="line">        <span class="keyword">this</span>.hash = hash;</span><br><span class="line">        <span class="keyword">this</span>.key = key;</span><br><span class="line">        <span class="keyword">this</span>.val = val;</span><br><span class="line">        <span class="keyword">this</span>.next = next;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>

<ul>
<li>Node数据结构很简单，从上可知，就是一个链表，但是只允许对数据进行查找，不允许进行修改</li>
</ul>
<h3 id="2-2-TreeNode"><a href="#2-2-TreeNode" class="headerlink" title="2.2 TreeNode"></a>2.2 TreeNode</h3><ul>
<li>TreeNode继承与Node，但是数据结构换成了二叉树结构，它是红黑树的数据的存储结构，用于红黑树中存储数据，当链表的节点数大于8时会转换成红黑树的结构，他就是通过TreeNode作为存储结构代替Node来转换成黑红树源代码如下</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">final</span> <span class="class"><span class="keyword">class</span> <span class="title">TreeNode</span>&lt;<span class="title">K</span>,<span class="title">V</span>&gt; <span class="keyword">extends</span> <span class="title">Node</span>&lt;<span class="title">K</span>,<span class="title">V</span>&gt; </span>&#123;</span><br><span class="line">    TreeNode&lt;K,V&gt; parent;  <span class="comment">// red-black tree links</span></span><br><span class="line">    TreeNode&lt;K,V&gt; left;</span><br><span class="line">    TreeNode&lt;K,V&gt; right;</span><br><span class="line">    TreeNode&lt;K,V&gt; prev;    <span class="comment">// needed to unlink next upon deletion</span></span><br><span class="line">    <span class="keyword">boolean</span> red;</span><br><span class="line"></span><br><span class="line">    TreeNode(<span class="keyword">int</span> hash, K key, V val, Node&lt;K,V&gt; next,</span><br><span class="line">             TreeNode&lt;K,V&gt; parent) &#123;</span><br><span class="line">        <span class="keyword">super</span>(hash, key, val, next);</span><br><span class="line">        <span class="keyword">this</span>.parent = parent;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>





<h3 id="2-3-TreeBin"><a href="#2-3-TreeBin" class="headerlink" title="2.3 TreeBin"></a>2.3 TreeBin</h3><ul>
<li>TreeBin从字面含义中可以理解为存储树形结构的容器，而树形结构就是指TreeNode，所以TreeBin就是封装TreeNode的容器，它提供转换黑红树的一些条件和锁的控制，部分源码结构如下</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">final</span> <span class="class"><span class="keyword">class</span> <span class="title">TreeBin</span>&lt;<span class="title">K</span>,<span class="title">V</span>&gt; <span class="keyword">extends</span> <span class="title">Node</span>&lt;<span class="title">K</span>,<span class="title">V</span>&gt; </span>&#123;</span><br><span class="line">    TreeNode&lt;K,V&gt; root;</span><br><span class="line">    <span class="keyword">volatile</span> TreeNode&lt;K,V&gt; first;</span><br><span class="line">    <span class="keyword">volatile</span> Thread waiter;</span><br><span class="line">    <span class="keyword">volatile</span> <span class="keyword">int</span> lockState;</span><br><span class="line">    <span class="comment">// values for lockState</span></span><br><span class="line">    <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> WRITER = <span class="number">1</span>; <span class="comment">// set while holding write lock</span></span><br><span class="line">    <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> WAITER = <span class="number">2</span>; <span class="comment">// set when waiting for write lock</span></span><br><span class="line">    <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> READER = <span class="number">4</span>; <span class="comment">// increment value for setting read lock</span></span><br></pre></td></tr></table></figure>



<p><a href="https://www.jianshu.com/p/865c813f2726" target="_blank" rel="noopener">https://www.jianshu.com/p/865c813f2726</a></p>
<p><strong>未完待续 ： 等我有空接着写。</strong></p>
<p><strong>参考博客</strong> ：  <a href="https://www.jianshu.com/p/865c813f2726" target="_blank" rel="noopener">https://www.jianshu.com/p/865c813f2726</a></p>

      
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